Package

Large systems are often divided into smaller units, called packages, in order to make them more manageable.  

In Visual Case, you can drop a package object onto any UML diagram.  Then, using the right-click menu, you can explode the package into a new UML diagram of any type.

Classes, interfaces, and other packages can be assigned to the contents of a package.  The property pages of these objects show which package they belong to.